Russian President Vladimir Putin accused Turkey on Monday of shooting down its warplane last week to protect its oil trade with the Islamic State.

Turkey has maintained, since the incident happened last Tuesday, that the Russian bomber crossed into Turkish airspace; and after issuing repeated warnings to leave, the plane was shot down. The U.S., after reviewing the facts available, has sided with the Turkish version of events.

Turkish Prime Minister Ahmet Davutoglu stated, “The protection of our land borders, our airspace, is not only a right, it is a duty.” He added: “We apologize for committing mistakes, not for doing our duty.”

The country’s president, Recep Tayyip Erdogan, said he would resign his office if Russia can prove its claim regarding the purchasing of oil from ISIS.

“As soon as such a claim is proved, the nobility of our nation requires (me) to do this,” Erdogan told reporters at the climate change summit in Paris on Monday, according to CNN.

Erdogan added that if Putin’s claim is false, he should resign. “I am asking Mr. Putin, would you remain?”

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Putin said at a press conference in Paris Monday, “We have recently received additional reports that confirm that that oil from ISIL-controlled [Islamic State of Iraq and the Levant – former name of the Islamic State terrorist organization] territories is delivered to the territory of Turkey on an industrial scale,” the Russian news agency TASS reported.

“We have all grounds to suspect that the decision to down our plane was motivated by the intention to secure these routes of delivering oil to ports where it is loaded on tankers,” Putin added.

“It is obvious where we legally buy oil and natural gas from,” Erdogan said, as reported by Turkey’s Anadolu news agency. “Everyone must know that we are not that disreputable to make such a deal with terrorist organizations.”

The BBC’s David Butter questions whether Putin actually believes his charges, but thinks he is likely using the standoff for propaganda purposes to legitimize hitting non-ISIS, Turkish-backed Syrian rebel targets. Turkey opposes Syria’s Assad regime, while Russia intervened in the civil war to prop it up.